This statement is objectively false. Not only do Lee's critics hold Marse Roberts responsible for the depredations of his forces in Maryland and Southern Pennsylvania, but Lee himself admitted to them and tried to justify them on the basis of similar tactics in use by Union forces in Virginia (which did not, in fact, occur.) Lee's rationalization that the gratuitous destruction in South Central Pennsylvania was what "is happening in Virginia every day," has been thoroughly demolished by objective historians. Furthermore, the diaries of many in Lee's army regarding the Sack of Chambersburg make it clear that even enlisted men objected to what was done, and many wanted no part of it.
Confederate Armies avoided contact with Union forces? Seriously? You must think the war was fought entirely in Northern Virginia. Here's a news flash for you: It wasn't. Southern historians have focused on the War in Northern Virginia because in many other theaters after 1861 the Confederates were beaten, often badly. Furthermore, as Mosby, Quantrill and Forrest amply demonstrated, the Confederates had no qualms about attacking Union targets -- especially if they were defenseless civilians.
What kept Lee from attacking Union forces was simply this: while brilliant in situations in which he could make use of defensive fortifications, defensive tactics, and interior lines, Lee was largely incompetent on the attack.
He got his ass thoroughly kicked at Malvern Hill, and after that slaughter learned nothing from it, choosing to repeat the same mistake under even more unfavorable circumstances at Gettysburg, with even more disastrous results.
"Lee's Perfect Battle" at Chancellorsville was entirely Stonewall's perfect battle; it would never have happened had not an overconfident Lee blundered into Hooker's trap, leaving him no choice but the suicidal decision to divide his forces in the face of a numerically superior enemy.
Lee's army preferred defense because their commander lacked the skill for large scale offensive operations, had no ability to supply his army over long distances, and because after Chancellorsville he had only one decent Corps Commander [James Longstreet.]
The claim that Lee's army never caused unnecessary damage or harm is laughable Southern revisionism. When Lee failed to do so, it was only because he lacked the means. He made two desperate attempts to project the War into Northern territory in force, at Antietam and Gettysburg. In both attempts, he failed. After that, he came to understand his limitations, even if Southern revisionists have not.